Thermal sprayed electrodes

ABSTRACT

A method for the synthesis of an electrode ( 1 ), and the resulting article ( 1 ) therefrom, comprising coating an active material feedstock ( 3 ) with an additive material suitable for preventing thermal decomposition of said feedstock ( 3 ) during thermal spray, thermal spraying the coated feedstock ( 3 ) onto a substrate ( 2 ) for an electrode, thereby forming a coating on the substrate ( 2 ), thereby providing an electrode ( 1 ).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the manufacture of porouselectrodes for energy storage devices and energy conversion devices bychemical and thermal spray techniques. In particular, this inventionrelates to the fabrication of thin film oxide and non-oxide electrodesby thermal spray. Such manufacture advantageously uses continuousprocesses suitable for high-volume electrode production.

[0003] 2. Description of the Related Art

[0004] Energy storage devices, such as batteries and supercapacitors,and energy conversion devices, such as fuel cells and thermoelectrics,both require electrodes comprising an active material for the energystorage, conversion, and/or release processes. Each year, billions ofdollars are spent on both primary and rechargeable batteries for use inapplications ranging from small batteries for portable electronics andcommunications equipment, to larger batteries used for automobiles anduninterruptible power supplies (UPS). Many of the industrialmanufacturing processes associated with the fabrication of theelectrodes containing active material (faradaic) are based on batchprocesses, often incorporating labor-intensive hand operations. Thereexists a critical need to develop continuous processes for electrodemanufacture that enable the production of low-cost electrodes for bothenergy storage and energy conversion devices.

[0005] There is especially a need for efficient manufacture of thin filmelectrodes (e.g. 10 mil or less), where conventional pressing techniquesare inappropriate for disk electrodes with diameter in excess of 2inches in the absence of a supporting substrate. Thin film electrodeshave been fabricated by various techniques, including spray pyrolysisand chemical vapor deposition (CVD). Spray pyrolysis is used in thepreparation of thin films comprising metal oxides. In spray pyrolysis, anegatively-charged substrate with a heating element to control thetemperature is provided, and a precursor solution with the proper molarratio is forced to flow through a positively charged nozzle onto thenegatively charged substrate. The spray droplets tend to move to the hotsubstrate, primarily due to electrostatic attraction, and pyrolysistakes place at or near the surface of the substrate. This technique hasbeen used to fabricate electrodes comprising LiCoO₂, LiMn₂O₄, yttriastabilized zirconia (YSZ).

[0006] Thin film electrodes have also been previously fabricated bychemical vapor deposition (CVD) and related techniques. A typical CVDprocess involves the steps of vaporizing precursors to the vacuumchamber; triggering reaction of the vaporized precursors; and depositingthe reaction product onto the surface of substrate. This basic processhas been used to fabricate electrodes comprising MoS₂ by conventionalCVD, ZrO₂-TiO₂-Y₂O₃ by laser CVD (wherein the laser is the heat sourceof the substrate and reaction activator), and TiS₂ by plasma CVD.

[0007] Thin film electrodes have also been prepared by sol-gel methods(CeO₂-TiO₂ electrodes), electrochemical method (amorphous MnO₂electrodes), and molecular beam deposition (γ-In₂Se₃). Most, if not allof the above-described processes have been limited to small-scaleelectrode production, and are further ill-suited to adaptation to acontinuous process suitable for low-cost, high volume production.

[0008] Recently, fabrication of electrodes by thermal spray has beendisclosed by U.S. Pat. No. 5,716,422 to Muffoletto et al., which isincorporated herein by reference. U.S. Pat. No. 5,716,422 teaches theuse of a variety of thermal spraying processes for depositing anelectrochemically active material onto a substrate, resulting in a thinfilm electrode. Suitable spraying techniques include chemical combustionspraying processes, for example powder flame spraying, and electricheating spraying processes, for example plasma spraying. Muffoletto etal.'s preferred electrochemically active materials include metals, metaloxides, mixed metal oxides, metal sulfides and carbonaceous compoundsand mixtures thereof. More particularly, the use of copper oxide, cobaltoxide, chromium oxide iron sulfide and iron disulfide is disclosed.

[0009] A significant drawback of the thermal spraying processes resultsfrom the thermal instability of some of the electrochemically activematerials, particularly iron disulfide (“pyrite”). Pyrite is thermallyunstable, decomposing to FeS at about 550° C., which is much cooler thanthe flame temperature of plasmal spray. Although certain well-knowntechniques can provide lower flame temperature, the oxidized nature ofthe flame (the flame consists of propylene and oxygen) prevents thepossibility of its application in the spraying of pyrite.

[0010] There accordingly remains a need for methods for producingelectrodes using thermal spraying and certain preferredelectrochemically active materials while avoiding thermal decompositionof said electrochemically active materials. Further, there exists a needfor a thermal spraying process wherein the electrochemically activematerials comprise nanostructured materials.

SUMMARY OF THE INVENTION

[0011] The above-discussed and other drawbacks and deficiencies of theprior art are overcome or alleviated by the method of the presentinvention, wherein thermal spray of active material feedstocks is usedto fabricate porous electrodes for energy storage devices and energyconversion devices. Active material feedstocks for thermal spray arereadily available by chemical synthesis in aqueous solution at lowtemperature (<90° C.). In an advantageous feature of the presentinvention, the active material feedstocks undergo a reprocessing stepwhereby they are uniformly coated with sulfur prior to thermal spray.The sulfur coating prevents thermal decomposition of the activematerials during the spraying process. Thermal spray methods functionwith a wide variety of active material feedstocks, and are readilyadaptable to continuous manufacturing processes. In another advantageousfeature of the present invention, the active material feedstockcomprises nanostructured materials, which after thermal spray results inelectrodes having nanostructured active materials.

[0012] The above-discussed and other features and advantages of thepresent invention will be appreciated and understood by those skilled inthe art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The FIGURE is a schematic diagram of a coated electrode 1 whereina substrate 2 is coated with an active material feedstock 3, inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] In accordance with the present invention, thermal spray of anactive material feedstock is used to fabricate porous electrodes. Activematerial feedstocks for thermal spray are readily available by chemicalsynthesis in aqueous solution at low temperature (<90° C.). The purposeof this invention is to enable low cost production of electrodes, eitherthick or thin film. This invention applies to the fabrication ofelectrodes using either conventional or nanostructured materials as afeedstock for thermal spray. In a particularly advantageous feature ofthe present invention, the active material feedstocks undergo areprocessing step whereby the materials are coated with one or moreadditives to suppress thermal decomposition during thermal spraying.

[0015] Active material feedstocks are selected from the group consistingof metals, metal oxides, mixed metal oxides, metal sulfides,carbonaceous materials and mixtures thereof, for example, nickelhydroxide and manganese dioxide. More preferably, the active materialfeedstocks are metal sulfides, in particular cobalt disulfide,molybdenum disulfide, and tungsten sulfide (WS₂). An especiallypreferred active material feedstock is iron disulfide (pyrite).Synthesis of thin films of pyrite has previously been investigated.Muffoletto et al. teaches the use of iron disulfide as an activematerial feedstock for deposit onto a substrate via thermal spray. G.Pimenta et al. have produced pyrite using H₂S-reactive iron. Pyrite andpyrite films have also been prepared by chemical vapor transportation,sulfurization of iron oxides, electrodeposition of iron films, argon andreactive sputtering, screen printing processes, and chemical vapordeposition. Conventional and fine pyrite (micron-sized) is also producedin aqueous solution.

[0016] Pyrite is thermally unstable, decomposing to FeS at about 550° C.The flame temperature of plasmal spray is thus much hotter than thedecomposition temperature of pyrite. Although HVOF can provide lowerflame temperature, the oxidized nature of the flame (the flame consistsof propylene and oxygen) prevents the possibility of its application inthe spraying of pyrite. Accordingly, appropriate steps must be taken toprevent the thermal decomposition of the pyrite.

[0017] There are at least two ways to suppress the decomposition, thefirst being to reduce the flame temperature by varying the sprayconditions. However, practice has shown that a reduction in flametemperature alone will not fully prevent pyrite decomposition.Consequently, a second method of preventing decomposition may be used inaddition to or alternatively to temperature reduction. This secondmethod involves the use of one or more additives to provide a protectivecoating surrounding the pyrite particles while in the spray gun flame.Preferred additives include cornstarch and sulfur, with sulfur beingmore preferred.

[0018] The coating step comprises mixing a quantity of additiveeffective to suppress thermal decomposition with the active materialfeedstock. The composition is mixed using methods known in the art untileach component is uniformly dispersed. Ball milling is a particularlyeffective mixing technique. The composition is then dried if necessary.The surface moisture of the active material feedstock is thereby removedand replaced with a coating comprising the additive. An appropriateadditive has relatively high melting and boiling points, accordingly,the coating slows the active material feedstock from heating andprevents thermal decomposition. Additional benefits of the coatinginclude much better flowability of the reprocessed powder and theability to store the powder outside of a vacuum.

[0019] By way of illustration, and in no way intended to be limiting, anexemplary additive material for the purpose of providing a protectivecoating is elemental sulfur powder. Sulfur has a melting point of 120°C. and boiling point of 440° C., both of which are below thedecomposition temperature of pyrite. The aforementioned reprocessingsteps provide a sulfur coating on the surface of pyrite particles. Assulfur is a poor heat conductor, the coating slows the heating of thepyrite powder. The coating consumes large amounts of energy,transforming the solid state sulfur to liquid and gas phases.

[0020] In another advantage of the present invention, heating the sulfurcoated pyrite powder to 550° C. results in the following decompositionreaction:

FeS₂⇄2FeS+S₂(g)

[0021] Above 440° C., S₂ gas forms around the solid pyrite particles,causing additional sulfur gas partial pressure. The additional pressureis a favorable condition for the reformation of pyrite. Consequently,the decomposition is further prevented. This is borne out by X-ray data,which show that without sulfur additive, the thin film comprises FeS,FeS_(x) (where x<2), Fe₂O₃, Fe₃O₄ and other undesired phases. Withsulfur additive, the primary phase of the thin film (>95%) is FeS₂.

[0022] In another particularly advantageous feature of the presentinvention, the active material feedstock comprises nanostructuredmaterials which, after thermal spray, results in electrodes withnanostructured active material. As used herein “nanostructured”materials refers to materials having a grain size on the order of 1 to100 nanometers (where 1 nm=10 angstroms). Nanostructured materials arethus characterized by having a high fraction of the material's atomsresiding at grain or particle boundaries. For example, with a grain sizein the five nanometer range, about one-half of the atoms in ananocrystalline or a nanophase solid reside at grain or particleinterfaces. Rapid interaction between the active materials and itssurroundings are possible because of high surface area of thenanostructured materials. Therefore, the materials could sustain highcurrent charging and discharging conditions. Thermal spray ofnanostructured feedstocks to produce a nanostructured coating isdisclosed in pending U.S. patent application Ser. No. 09/019061, filedFeb. 5, 1998, entitled “Nanostructured Feeds for Thermal Spray Systems,Method of Manufacture, and Coatings Formed Therefrom,” which is acontinuation of U.S. patent application Ser. No. 08/558,466 filed Nov.13, 1995, entitled “Nanostructured Feeds for Thermal Spray Systems,Method of Manufacture, and Coatings Formed Therefrom,” which isincorporated herein by reference. Synthesis of nanostructured materialsis disclosed in application Ser. No. 08/971,817 filed Nov. 17, 1997 byTongsan Xiao et al., entitled NANOSTRUCTURED OXIDES AND HYDROXIDES ANDMETHODS SYNTHESIS THEREFOR, which in incorporated herein by reference inits entirety.

[0023] Preferably, the active material feedstock, whether comprised ofone of the above mentioned materials or a similarly suitable material,is deposited onto a titanium or an aluminum substrate by one of thethermal spraying processes mentioned below.

[0024] Thermal spraying processes for use with the present invention arewell known in the art. Known spraying processes may be classified intotwo groups, namely, chemical combustion spraying processes and electricheating spraying processes. Chemical combustion spraying processesinclude powder flame spraying, wire/rod flame spraying, high velocityoxygen fuel flame spraying and detonation/explosive flame spraying.Electrical heating processes include electric-arc or twin-wire arcspraying and plasma spraying.

[0025] The invention is further illustrated by the followingnon-limiting examples.

EXAMPLE 1 Electrodes Comprising Pyrite

[0026] A. Reprocessing of Pyrite

[0027] About 20 grams of sulfur powder is mixed with 200 grams of pyritepowder and ball milled in a ceramic jar for 24 hours. The uniformlymixed powder is then placed in a vacuum oven and dried at 150° C. undervacuum for 12 hours. The surface moisture of pyrite is thereby removedand the surface of the pyrite coated by sulfur due to its low meltingpoint (˜120° C.). The treated powder has much better flowability, anddue to high dihedral angle between sulfur and water, the treated powderwas not required to be stored under a vacuum.

[0028] B. Apparatus

[0029] In general, a plasma gun is connected with a robot which has sixdimension movement, and the workpiece is fixed on the sample stage. Analternative apparatus consists of a stainless steel box having a frontcover accommodating a fixed plasma gun, a nitrogen gas inlet, and arobot connected to a sample holder. With this apparatus, the samplemoves instead of the plasma gun, which is sealed with rubber between thefront cover and the stainless steel box. With this apparatus, the oxygencontent inside the chamber could be reduced to less than 5%.

[0030] The adjustable parameters of the plasma gun include arc current,argon flow rate, and carrier gas rate. If desired, the flame temperaturecan be decreased by reducing the arc current, and increasing the argonflow and carrier gas rates. However, an over-cooled flame will not meltthe particle surfaces, resulting in a poor coating. A much preferredspray condition is achieved with 180A, and 250 SCFH argon flow.

[0031] C. Thermal Spray of Conventional (Micron and Greater-Sized)Pyrite Feedstock

[0032] Prior to thermal spray, the inert gas chamber is purged withnitrogen gas for 10 minutes. With the treated powder, coatings wereproduced using a Metco 9 MB plasma spray system. At about 150 Ampere and70 Volts, with 250 SCFH argon flow and 4 lb/hr feeding rate, electrodeswere sprayed at conditions 3 c and 5 b on 1.25 inch diameter 0.006 inchthick grit blasted stainless steel disks. At both 200 and 300 Amp 250SCFH argon conditions, the coating adhered well to the substrate, andwas not fragile during handling. Also, little curling occurredindicating low stresses and the potential to spray much thicker layers.

[0033] The experiments have been carried out in the inert gas chamberwith the best plasma spray conditions. The results show no evidence ofiron oxide present in the deposited films. The x-ray pattern alsoindicates that there are more iron sulfide left in the coating comparedto the one without the protection of an inert gas chamber.

EXAMPLE 2 Reprocessing and Thermal Spray of Nanostructured Pyrite

[0034] Nanostructured pyrite is synthesized by aqueous solution methodat low temperature (<90° C.) in relatively short period (2-4 hours).Synthesized nanostructured FeS₂ has particle size less than 100 nm.About 20 grams of sulfur powder was mixed with 200 gram of thenanostructured pyrite powder and ball milled in a ceramic jar for 24hours. Thereafter, the uniformly mixed powder is placed in a vacuumoven, and dried at 150° C. under vacuum for 12 hours. The treated powderthen is dispersed in 10% PVA solution and the suspension is then spraydried at 200° C. in accordance with U.S. Ser. No. 08/553,133 above. Theparticle size of reprocessed powder is in the range of 1-200 nm. Thethin film electrode of pyrite was fabricated with the plasma spray byMetlco 9 MB plasma gun as described above to form a nanostructure pyriteelctrode.

EXAMPLE 3 Thermal Spray of Ni(OH)₂

[0035] The thin film electrode of Ni(OH)₂ was fabricated with the plasmaspray by Metlco 9 MB plasma gun. The arc current is 120A at 70 V withargon flow about 250 SCFH. The Ni(OH)₂ powder was feed at rate of 2lb/hr with 70 SCHF carrier gas.

EXAMPLE 4 Thermal Spray of Nanostructured Ni(OH)₂

[0036] Nanostructured Ni(OH)₂ is reprocessed by spray-during theas-synthesized powders to agglomerates in accordance with U.S. Ser. No.08/553,133 above. The thin film electrode of nanostructured Ni(OH)₂ isfabricated by Metlco 9 MB plasma gun. The arc current is 120A at 70 Vwith argon flow about 250 SCFH. The Ni(OH)₂ powder was feed at rate of 2lb/hr with 70 SCHF carrier gas.

EXAMPLE 5 Thermal Spray of MnO₂

[0037] A thin film of MnO₂ was fabricated with the plasma spray byMetlco 9 MB plasma gun. The arc current is 200A at 70 V with argon flowabout 200 SCFH. The MnO₂ powder is a feed at rate of 3 lb/hr with 70SCFH carrier gas.

EXAMPLE 6 Thermal Spray of Nanostructure MnO₂

[0038] Nanostructured MnO₂ is reprocessed as in Example 3. The thin filmelectrode of nanostructured MnO₂ is fabricated under the same conditionas Example 4.

[0039] While preferred embodiments have been shown and described,various modifications and substitutions may be made thereto withoutdeparting from the spirit and scope of the invention. Accordingly, it isto be understood that the present invention has been described by way ofillustration and not limitation.

What is claimed is:
 1. A method for the synthesis of an electrode,comprising coating an active material feedstock with an additivematerial suitable for preventing thermal decomposition of said feedstockduring thermal spray; and thermal spraying the coated feedstock onto asubstrate for an electrode, whereby forming a coating on the substrate,to provide an electrode.
 2. The method of claim 1, wherein the electrodeis a thin film electrode.
 3. The method of claim 1, wherein the additivematerial is elemental sulfur.
 4. The method of claim 1, wherein theadditive material is cornstarch.
 5. The method of claim 1, wherein thesubstrate comprises titanium or aluminum or mixtures thereof.
 6. Themethod of claim 1, wherein the active material feedstock is a metal, ametal oxide, a mixed metal oxide, a metal sulfide, a carbonaceousmaterial, and mixtures thereof.
 7. The method of claim 1, wherein theactive material feedstock is silver vanadium oxide, copper silvervanadium oxide, manganese dioxide, copper oxide, chromium oxide, cobaltoxide, nickel oxide, nickel hydroxide, titanium disulfide, coppersulfide, iron sulfide, iron disulfide, cobalt disulfide, molybdenumdisulfide, tungsten sulfide, carbon, fluorinated carbon, and mixturesthereof.
 8. The method of claim 1 wherein the active material feedstockcomprises pyrite.
 9. The method of claim I wherein the active materialfeedstock is nanostructured.
 10. The method of claim 9 wherein theelectrode coating is nanostructured.
 11. The method of claim 8 whereinthe active material feedstock is nanostructured.
 12. The method of claim11 wherein the electrode coating is nanostructured.
 13. The method ofclaim 1, wherein the thermal spraying is carried out in an inertatmosphere.
 14. The method of claim 1, wherein the thermal spraying iscarried out in an atmosphere comprising less than about 5% oxygen. 15.The method of claim 1, wherein thermal spraying is by plasma gun.
 16. Anelectrode manufactured by the method of claim
 1. 17. The electrode ofclaim 16, wherein the electrode is a thin film electrode.
 18. Theelectrode of claim 16, wherein the additive material is elementalsulfur.
 19. The electrode of claim 16, wherein the additive material iscornstarch.
 20. The electrode of claim 16, wherein the substratecomprises titanium or aluminum or mixtures thereof.
 21. The electrode ofclaim 16, wherein the active material feedstock is a metal, a metaloxide, a mixed metal oxide, a metal sulfide, a carbonaceous material,and mixtures thereof.
 22. The electrode of claim 16 wherein the activematerial feedstock is silver vanadium oxide, copper silver vanadiumoxide, manganese dioxide, copper oxide, chromium oxide, cobalt oxide,nickel oxide, nickel hydroxide, titanium disulfide, copper sulfide, ironsulfide, iron disulfide, cobalt disulfide, molybdenum disulfide,tungsten sulfide, carbon, fluorinated carbon, and mixtures thereof. 23.The electrode of claim 16 wherein the active material feedstockcomprises pyrite.
 24. The electrode of claim 16 wherein the activematerial feedstock is nanostructured.
 25. The electrode of claim 24wherein the electrode coating is nanostructured.
 26. The electrode ofclaim 23 wherein the active material feedstock is nanostructured. 27.The electrode of claim 26 wherein the electrode coating isnanostructured.
 28. The electrode of claim 16, wherein the thermalspraying is carried out in an inert atmosphere.
 29. The electrode ofclaim 16, wherein the thermal spraying is carried out in an atmospherecomprising less than about 5% oxygen.
 30. The electrode of claim 16,wherein thermal spraying is by plasma gun.